The switched mode power supply (SMPS) is a well-known type of power converter having a diverse range of applications by virtue of its small size and weight and high efficiency, for example in personal computers and portable electronic devices such as cell phones. An SMPS achieves these advantages by switching a switching element such as a power MOSFET at a high frequency (usually tens to hundreds of kHz), with the frequency or duty cycle of the switching being adjusted using a feedback signal to convert an input voltage to a desired output voltage. An SMPS may take the form of a rectifier (AC/DC converter), a DC/DC converter, a frequency changer (AC/AC) or an inverter (DC/AC).
FIG. 1 is a simplified circuit diagram of a switched mode DC/DC power supply 10 in the form of a buck converter which converts an input voltage Vin to a desired output voltage Vout, which is applied across a load represented by a resistor 20 connected in parallel with a load capacitor 30. The power supply 10 comprises an inductor 40, a filter capacitor 50, a diode 60, a power transistor 70 and a pulse-width modulating (PWM) controller 80, which controls the operation of the power supply. Although a single filter capacitor 50 is shown in the simplified diagram of FIG. 1, switched mode power supplies typically use large banks of capacitors for maintaining a stable output voltage during load transients or for keeping the voltage ripple at an acceptable level. The PWM controller applies voltage pulses 90 at an appropriate frequency (e.g. 300 kHz) to the gate of the power transistor 70. The PWM controller regulates the output voltage Vout by adjusting the duty cycle D of the pulses (defined by D=TON/Tswitch, where TON is the duration of a pulse and Tswitch is the switch period) on the basis of a feedback signal generated by a differential amplifier 100. The feedback signal is indicative of the difference between the output voltage Vout and the amplifier's reference voltage, VRef, which is controlled by the controller 80.
In order to optimise the performance of the feedback loop comprising the PWM controller 80, the system needs to be properly identified. A common method in system identification is to superimpose a disturbance on the normal signal and analyze what happens on the output, for example as described in “System Identification—Theory for the User” by L. Ljung (Prentice-Hall, Englewood Cliffs, ISBN 0-13-B81640, 1987). This disturbance can be injected in many different ways. One way is to use relay feedback, as described in “Automatic tuning of PID controllers” by K. J. Åström and T. Hagglund (Instrument Society of America, ISBN 1-55617-081-5, 1988). Alternatively, the disturbance can be injected by causing limit cycles or injecting a noise signal, as described in the doctoral thesis of Zhao Zhenyu, entitled “Design and Practical Implementation of Digital Auto-tuning and Fast-response Controllers for Low-power Switch-mode Power Supplies” (University of Toronto, Canada, 2008).
One critical problem with the above schemes is to control or limit the amplitude of the disturbance at the output. The loads of the SMPS sometimes have very strict requirements on overvoltage, which make this method inappropriate or even impossible to use. Furthermore in some applications, particularly those in the telecommunications industry, no disturbance at all on the output voltage can be tolerated. The aforementioned methods employing feedback control require complex calculations that take time and consume power. In addition, these methods place heavy demands on computational resources. Other methods which involve injecting sinusoidal disturbances and using cross-correlation in order to determine the system's transfer function are also time-consuming and require complex calculations that takes time and consume power, and require a lot of computation resources to be allocated.
It is therefore highly desirable to develop a scheme for optimising the feedback loop parameters in an SMPS that avoids the injection of any disturbance which can cause additional noise on the output voltage of the power supply. This requires a measure of the total capacitance coupled to the power supply's output to be established. This capacitance affects the system dynamics of the SMPS and must be considered during control law synthesis.
The development of new CMOS technologies in loads such as ASICs and FPGAs has led to tougher requirements on the power supply's current capabilities and voltage tolerance bands. Moreover, a bank of capacitors with a mix of different capacitor types is now often used for optimizing the electrical performance and cost of a particular load circuit. In addition, technical development has improved the electrical characteristics of the capacitors such that they have a higher capacitance and a lower equivalent series resistance (ESR). Thus the capacitive load can vary widely between different applications.
Accordingly, in situ identification of the capacitive load is of great interest for obtaining a good model of the system dynamics. Identification can also be used in combination with some autotuning algorithm which makes it possible to adjust for aging components and temperature drift in the real application, for example.